Fix RAM issue

[oke-aditya]

We should lazy load models, this is major fix for memory leak.

I will also need to do release 0.2.1 for this

[codecov]

Codecov Report

Merging #102 (f938101) into master (0a29870) will increase coverage by 0.21%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
+ Coverage   77.14%   77.35%   +0.21%     
==========================================
  Files          50       50              
  Lines        1584     1568      -16     
==========================================
- Hits         1222     1213       -9     
+ Misses        362      355       -7     

Flag	Coverage Δ
unittests	77.35% <80.00%> (+0.21%)	⬆️

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Impacted Files	Coverage Δ
quickvision/pretrained/_model_zoo.py	100.00% <ø> (ø)
...kvision/models/components/torchvision_backbones.py	90.38% <75.00%> (+8.29%)	⬆️
...kvision/models/classification/cnn/model_factory.py	100.00% <100.00%> (ø)

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